Flexography is a direct rotary printing method which employs embossed plates made of photo-polymeric materials. The plates are flexible and soft, hence the name flexography. Such plates are inked and the print is obtained by means of direct deposition of the ink on the medium to be printed by virtue of a slight pressure exerted by a printing cylinder on which the plates are positioned.
Flexography is a high-speed printing process, capable of printing on many types of absorbing and non-absorbing materials. Some typical applications of flexographic printing are the production of paper and plastic bags, milk cartons, disposable cups and the like, but by virtue of the progress in printing quality, flexography is today also used for printing newspapers, for example, as well as packets and labels and for printing on plastic, films and acetate sheets, wrapping paper and on many other materials used for product packaging.
As known, the pre-printing step, i.e. the step of preparing the flexographic plate, which undergoes various steps, one of which is the step of exposing the plate to ultraviolet light (UV) to photo-polymerize it, is very important and delicate.
In order to obtain the required embossed characters, the plate is polymerized only at the points of a mask left permeable to light. In the case of the so-called analog printing plates, a negative film is applied onto the plate which shows the image to be printed in negative, while in the case of the so-called digital printing plates, the plates are covered with a layer which is abraded by means of laser engraving to create the negative of the image to be printed.
Exposing the plate to the light causes the polymerization only of the parts of the plate at the points permeable to light of the negative film, in the case of analog printing plates or of the points in which the layer of coating material was removed in the case of digital printing plates, thus forming the relief image to the printed on the polymer.
In all cases, the plate must be exposed to ultraviolet light (UV) for the necessary formation of the base (back exposure, i.e. of the bottom or back of the plate). In the apparatuses of the type known in the prior art, the steps of main exposure, i.e. exposing the main face of the plate, and of back exposure are generally performed in the scope of a continuous treatment process of the printing plate. After the exposure step, the flexographic plate undergoes other processing steps, including the washing step, by means of which the non-polymerized material is removed from the plate.
It has been seen that the integration in the exposure of the plate, above all with respect to the back exposure, in a continuous treatment process of a plate, determines a considerable complication in terms of design and control. At the same time, it has been seen that the inspection, maintenance and replacement operations of the means used for photo-polymerization are particularly complicated in a continuous process.
Thus, it is a main task of the present invention to provide an apparatus and method for the photo-polymerization of flexographic digital printing plates which allows to overcome the drawbacks mentioned above. In the scope of this task, it is a first object of the present invention to provide a compact, autonomous apparatus capable of autonomously carrying out at least the photo-polymerization of the secondary back face. In the scope of this task, it is another object of the present invention to provide an apparatus capable of autonomously carrying out the photo-polymerization of the main face and of the secondary back face. It is a further object of the present invention to provide an apparatus which is reliable and easy to be manufactured at competitive costs.